Temperature is an important variable in many industrial processes. Most processes can operate effectively in a relatively wide temperature window. Some processes however require extremely precise and reliable temperature control.
For example, semiconductor fabrication requires tight temperature control on many processes. Semiconductor wafers must be maintained at high temperatures with a very uniform temperature profile across the wafer surface. Excessive temperature variability can result in process yield loss. Details with regard to such representative heating application are evidenced with reference to, among other things, Applicant's earlier work, namely, US Pub. No. US 2006/0289447 A1 entitled HEATING CHUCK ASSEMBLY, incorporated herein by reference in its entirety.
High temperature heating chucks, e.g., sandwiched pedestal assemblies, generally comprise two hermetically sealed metal (e.g., aluminum, stainless steel, nickel/nickel alloy) or ceramic discs which house a combination of elements or a subassembly, including, but not limited to a heater, and more often than not, a heater characterized by multiple, independently operable/controllable heating zones. Commonly, but not exclusively, heaters may be characterized by rods, wires, etched foils and/or etched foil laminates characterized by a dielectric, e.g., mica, ceramic paper, Kapton® polyimide, silicon rubber, etc., with such heater assemblies further characterized by combinations of sensors, controllers, cabling and other electrical and/or mechanical components, as well as tight dimensional tolerances, surface flatness, perpendicularity, and a select surface finish.
As is to be expected, temperature specifications and tolerances of heating chucks are a function of the wafer process, e.g., chemical vapor deposition (CVD), plasma enhanced chemical vapor deposition (PECVD), lithography, baking, plasma etching, cleaning, etc. Generally, thin flexible heaters or heating elements (e.g., Thermofoil™ etched foil heaters from Minco Products, Inc., MN, USA) are advantageously utilized in heating chuck assemblies. Characteristic heaters available for the semiconductor industry are generally twofold, namely, low temperature, i.e., up to about 260° C., rod-type, polyimide or ceramic heaters, or high temperature, i.e., up to about 600° C., rod-type, mica, or ceramic heaters.
Wafers whose diameters are 200 mm and 300 mm are most pervasive, with 450 mm wafers emerging, however, as heating chucks become larger, it becomes more difficult to control the thermal tolerances during wafer processing. As a result, problematic thermal warping of the heating chuck remains an issue.
The thermal process control of both the wafer and the heating chucks are critical to wafer processing as operating temperatures are generally controlling, e.g., operating temperatures dictate, among other things, reaction kinetics of the chemical reactions of the wafer process. During such processes, layers of gases or thin films are deposited to form a solid insulating or conducting layer on the surface of a wafer. The gases react with material on the substrate thereby creating a thin film that has desirable electrical properties. High-quality films are those with a uniform chemical composition and thickness across the entire substrate area. The thermal process controls the density of the thin film deposited, which is also crucial to the overall wafer quality. Thus, in the interest of improving process yield, there is a desire for an improved heating system that can provide, among other things, greater temperature control (e.g., more independent control zones), and greater temperature uniformity across the wafer surface.
Current heater designs have several fundamental limitations that make performance improvement goals difficult, if not impossible. Limitations and/or challenges have origins in both the heating element of the assembly, the housing thereof, and the relationship for, between and among these primary assembly elements.
Rod-type heaters are large and have flexibility constraints which limit the uniformity of the heating coverage (i.e., there will always be large gaps somewhere in the heating element layout). These same constraints make a rod-heater based high zone count heating system impractical; there isn't enough room to route several pairs of rod-heater ends to and through a chuck/assembly stem or pedestal. Moreover, rod-heaters are not readily heat profiled (i.e., different sections outputting different amounts of power). Although not as limited, wire type heaters are characterized by similar shortcomings.
Etched foil heaters are superior to rod/wire heaters in applications that require temperature uniformity on a flat surface. Etched foil heaters cover a much higher percentage of the heated area and can be profiled to apply more heat near certain features (e.g., holes, edges, etc.). They are also easily made into multi-zone designs.
Be that as it may, etched foil heaters have one major drawback, namely, the electrical insulation (i.e., dielectric) that carries/surrounds the heating element (i.e., the dielectric material/matter interposed between, among, around the foil tracing so as to carry and cover the foil tracing). This electrical insulation layer is also a relatively good thermal insulator which creates a thermal break between opposing housing portions of the assembly. The thermal break permits and commonly results in the housing portions to exhibit dissimilar/disparate thermal profiles/temperatures due to non-uniform thermal loads which are known to contribute to housing warpage due to thermal expansion differences associated with the opposing housing portions.
Thermal properties of assemblies characterized by metal housings and/or plates or the like create a variety of well known material science tensions, note, e.g., the characterization of the problem by Mashima/Mashima et al., US Publ. Nos. 2006/0157472 A1 & 2006/0199135 A1, each of which is incorporated herein in their entireties by reference. Moreover, stainless steel, which enjoy widespread use in semiconductor heat processing, is generally a poor heat conductor. Without multiple independent temperature control zones, unwanted temperature gradients develop, either due to non-uniform heat input or not-uniform heat loss.
While mix and match approaches with regard to material selection in connection to heating and conducting functions are well known and documented, as are numerous approaches to maintain a fixed state for heater assembly elements in furtherance of integrity maintenance, an unmet need for a best of all worlds thermal solution remains for an assembly suitable for controlling the temperature of a workpiece. More particularly, it is believed advantageous and desirable to provide a heater cartridge characterized by an especially efficient heat conducting casing within which operably resides a composite heating element, more particularly, to provide such cartridge wherein interior casing portions are selectively united so as function as heat spreaders, especially in the context of a heater assembly characterized by a stainless steel housing.